The present invention relates to a preamplifying circuit that amplifies signals detected by a magnetoresistance device for a magnetic head.
As the recording density required for hard disks becomes ever higher, higher performance for magnetic heads is much desired and the use of a magnetoresistance device in the magnetic head is regarded as a very promising possibility. Since the signals detected by a magnetoresistance device are weak, the need for a preamplifying circuit with low noise and high gain has arisen.
FIG. 8 shows a prior art preamplifying circuit for a magnetoresistance device.
A signal detected by magnetoresistance device 10 is amplified in the differential amplification circuit 20. Since it is necessary to run a continuous bias current through the magnetoresistance device 10, one of its terminals is connected to the ground line via the constant current source 11. The other terminal of the magnetoresistance device 10 is connected to the power source line Vcc via the resistor 12. This resistor 12 is a means for applying a bias voltage to the base of the transistor 21 of the differential amplification circuit 20. The magnetic detection circuit is comprised of the resistor 12, the magnetoresistance device 10 and the constant current source 11.
In the differential amplification circuit 20, the transistor 21 is connected at its collector through the resistor 22 to the power source line Vcc, at its base to one terminal of the magnetoresistance device 10 and at its emitter through the constant current source 23 to the ground line. Likewise, the transistor 24 is connected at its collector through the resistor 25 to the power source line Vcc, at its base to the other terminal of the magnetoresistance device 10 and at its emitter through the constant current source 26 to the ground line. A capacitor 27 is connected between the emitter of the transistor 21 and the emitter of the transistor 24 to AC-couple them. A pair of output voltages *Vo and Vo are taken from the collectors of the transistor 21 and 24, respectively, and are further amplified in a differential amplification circuit (not shown).
With the above structure, since the current amplification factor h.sub.FE of the transistor 21 is sufficiently large and the base currents of the transistor 21 and 24 are insignificant in comparison with the lead-in current I.sub.1 of the constant current source 11, a constant electric current I.sub.1 runs through the magnetoresistance device 10. When magnetic flux transmitted through the magnetoresistance device 10 changes, the resistance of the magnetoresistance device 10 changes by .DELTA.R and with this, the voltage between the terminals in the magnetoresistance device 10 changes by .DELTA.Vi=I.sub.1 .DELTA.R and this is amplified in the differential amplification circuit 20.
In FIG. 8, apart from the magnetoresistance device 10, all the components 11-26 and the above-described differential amplification circuit (not shown in the figure) are constituted as an integrated semiconductor circuit.
Since the detected signal in the magnetoresistance device 10 is weak, the external noise 50 presents a problem. This external noise 50 is added to one terminal of the magnetoresistance device 10 via the parasitic capacitance 51 and is also added to the other terminal of the magnetoresistance device 10 via the parasitic capacitance 52. The parasitic capacitances 51 and 52 are regarded as having the same value and, therefore, the external noise 50 is applied to the base of the transistor 21 and to the base of the transistor 24 and is considered to be in-phase. As the differential amplification circuit 20 amplifies the difference between a pair of input signals, it does not amplify this in-phase signal.
However, as input impedance of one terminal of the magnetoresistance device 10 is different from that of the other terminal, the external noise added to the base of the transistor 21 and the external noise added to the base of the transistor 24 are not in phase, resulting in a reduced signal-to-noise (SN) ratio when amplified in the differential amplification circuit 20. Also, as the current input end of the constant current source 11 is connected to the base of the transistor 24, the noise from the constant current source 11 is also amplified in the differential amplification circuit 20, causing the SN ratio to be further reduced.